1. Field of the Invention
The present invention relates to a magnetic head and a method for the production thereof and more particularly to a magnetic head of the type in which a pair of core halves which are mainly made of magnetic oxide materials and at least one of which has a magnetic-gap-defining surface coated with a thin metal magnetic film are disposed in opposed relationship leaving a magnetic gap therebetween and a method for the production thereof.
2. Description of the Prior Art
Recently, magnetic recording media are increasingly intensively magnetized with an increase of the data packaging density. Therefore, in order to be compatible with magnetic media having such a high degree of coercive force, the core materials of the magnetic heads are increasingly made of Fe-Al-Si system alloys, amorphous magnetic materials such as Co-Zn-Nb systems and the like all having a high degree of magnetic flux density. In general, such metallic magnetic materials exhibit a high degree of high-frequency losses due to eddy current so that when a magnetic circuit of a magnetic head is composed only of such metallic magnetic materials, the reproduce efficiency drops in the high-frequency range. As a result, in order to compensate for high-frequency losses, there has been proposed a method for establishing a magnetic circuit by a combination of magnetic oxide materials having a high degree of high-frequency characteristics such as ferrites and metallic magnetic materials.
For instance, a magnetic head having a construction as shown in FIGS. 1A-1C recently has been proposed. FIG. 1A is a perspective view of a conventional magnetic head of the type described above; FIG. 1B shows a sliding surface for a magnetic recording medium; and FIG. 1C is a composite sectional view illustrating a degree of wear of the magnetic head.
This magnetic head comprises a pair of magnetic core halves 30 and 31 which are mainly made of ferromagnetic oxide materials such as a Mn-Zn ferrite (to be referred to as "ferrites" hereinafter in this specification). Magnetic-gap forming surfaces of magnetic core halves 30 and 31 are coated with ferromagnetic metallic thin films 32 and 33 such as Sendust, Permalloy or the like by a vacuum evaporation process. Such a pair of magnetic core halves 30 and 31 are joined with each other by a molten glass 34.
In this magnetic head, a magnetic circuit in the vicinity of a magnetic gap g is established by ferromagnetic metallic thin films having a high degree of saturation magnetic flux density so that the magnetic head exhibits sufficient recording characteristics when used with recording media such as metal tapes or the like having a high degree of coercive force. Furthermore, because almost all portions of the main magnetic circuit are established by a ferrite having a high degree of high-frequency characteristics so that high efficiency reproducibility can be attained in a high-frequency range.
However, in the magnetic head described above, different ferromagnetic thin films 32 and 33 are deposited upon the ferrite as a core material, a difference in the coefficient of expansion between the thin metallic films and the ferrite core arises due to the thermal treatment steps such as a step for joining two core halves with a molten glass and the like and in the step for depositing the thin metal films, adverse effects such that residual stresses and the like also arise. As a result, a so-called false or artificial gap is produced so that the frequency characteristic of the magnetic head tends to fluctuate.
In order to overcome the above-described problems, some measures have been proposed and demonstrated: for instance, a measure of a Co-Zn-Nb system amorphous alloy film whose coefficient of expansion is substantially similar to that of the ferrite core is used, and a measure of the step for joining the two core halves with a molten glass is carried out at low temperature so that thermal distortions due to the difference in coefficient of thermal expansion are reduced to a minimum as practically as possible. The above-mentioned production methods have a common step for using glass having a low melting point which must be used when the two core halves are joined together. In the case of amorphous metal thin films, the glass melting temperature is limited because of the limit of the crystallization temperature and in general glasses having a melting temperature lower than 500.degree. C. are used.
As described above, in the case of the magnetic head having a high degree of reproducibility when used with the above-mentioned recording media having a high coercive force; that is, in the case of the magnetic head in which a pair of core halves made of a ferrite have the magnetic-gap defining surfaces coated with a thin ferromagnetic metallic film, the butt-welding temperature for joining the core halves with a molten glass is limited in order to avoid adverse effects such as the false or artificial gap. Therefore, glass which has a low melting point and insufficient transportability and a low degree of resistance to adverse environmental effects must be used.
As a result, in the case of the magnetic head of the type as shown in FIG. 1A, wear of the glass 34 having a low melting temperature and exposed to a sliding surface of a recording medium is increased as compared with wear of the ferrite core halves 30 and 31. When the cross sections a and b of the sliding surface configurations are compared after the recording medium has been transported over the magnetic head for a long period of time, the cross section as shown in FIG. 1C is obtained so that the difference in wear becomes apparent. That is, at the cross section b shown in FIG. 1C, wear of the glass portion is fast and accordingly wear of the track portion (tw) of the center ferrite tends to increase so that the recording-medium-sliding surface is lower than that at the section a. Therefore, the portion adjacent to the magnetic gap portion g of the recording-medium-sliding surface becomes lower than the adjacent upstream and downstream portions. In addition, when the glass portions 34 which are made in contact with both ends of the magnetic gap drops, variations in spacing between the recording medium and the magnetic gap g tend to increase so that the reproduced envelop output tends to become unstable. Furthermore, the transportation of a recording medium tends to cause damages to a glass having a low melting point so that dust tends to adhere to damaged portions, resulting in the clogging of the magnetic gap.
As to the resistance to environmental condition of glass having a low melting temperature, water resistance becomes a problem. Therefore, due to the elution of Pb and Na/ from the glass the stepped portions are produced, the quality of the glass changes and discoloration results. As a result, variations in spacing between the recording medium and the recording gap tend to be increased.